fstab type "dp"

Hi,

A year ago, I discussed with rwatson_at_ about having a new fs_type
for dump devices.  I initially implemented it as "du" but I later
noticed that NetBSD already has this feature as "dp".

I'd like to get some review about this patch in the hope of getting this
patch eventually committed.

This patch contains:
- New fs_type "dp" for dump devices, along with "sw" for swap devices.
- dumpon(8) and savecore(8) updated accordingly.
- New dumpon(8) option -a that implements the logic to automatically
  select a correct dump devices by scanning fstab(5).   The logic is
  backward compatible, that is if no "dp" entry is present, it will use
  the first "sw" entry.  Also, etc/rc.d/dumpon has been modified to use
  this new option.
- New dumpon(8) option -f to force the use of a device not marked as
  "dp" or "sw".  This is a bonus.
- Manpages updated.

Note that both dumpon(8) and savecore(8) required the fs_vfstype to be
"swap" for "sw" devices for "XXX backward compatibility".  Despite this
wasn't documented in the manpage, I didn't removed it and added "dump"
as a valid fs_vfstype.  The fstab(5) manpage still doesn't mention them
though.

Thank you.
Best regards,
-- 
Jeremie Le Hen
< jeremie at le-hen dot org >< ttz at chchile dot org >